1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a pattering device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a pattering device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
In an apparatus as described above it is very important that the substrate table, which holds the substrate, and the support structure which may hold the mask is positioned with a very high precision. It is therefore proposed, for example, in U.S. Pat. No. 5,120,034 to use a positioning device comprising a carriage displaceable in at least two coordinate directions with respect to a base and intended for displacement of a table which is displaceable with respect to the carriage in the two coordinate directions wherein the table is coupled to the carriage solely by Lorentz forces of the magnet systems and coil systems in the two coordinate directions.
A critical issue in the use of Lorentz actuators in lithographic projection apparatus is the amount of heat generated in use by the coils of the Lorentz actuator, especially any heat that is produced by a Lorentz actuator mounted very close to the substrate table or the support structure. Any heat produced must be dissipated in order to prevent temperature changes in the substrate table and the support structure that could lead to losses in the accuracy of the apparatus.
Lorentz actuators include an electrically conductive element, such as a coil, and a magnet assembly. The magnet assembly produces a magnetic field which interacts with a current flowing in the electrically conductive element to produce a Lorentz force between the electrically conductive element and the magnet assembly in a direction perpendicular to the direction of the current flow and the magnetic field at that point. Typically the magnet assembly is comprised of at least one magnet on either side of the electrically conductive element to produce an approximately uniform magnetic field around the electrical conductor. Lorentz actuators do not include any iron in the coils. The magnet assembly of a Lorentz actuator does, however, comprises a back iron, formed from a material with high magnetic saturation, located on the outward side of the magnets. The back iron is typically required to be large to prevent saturation and it constitutes a substantial part of the mass of the actuator and is a source of loss of efficiency in the motor.
It is an aspect of the present invention to provide a lithographic projection apparatus with a Lorentz actuator having a back iron of reduced mass but no loss of performance of the actuator.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to supply a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table that holds a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; and a motor comprising a long stoke motor constructed and arranged to produce a force between a first part of the motor and a machine frame of the apparatus over a long range of movement in at least one direction and a Lorentz actuator constructed and arranged to produce a force between the first part and a second part of the motor over a short range of movement in at least one direction, the second part being connected to the support structure or the substrate; a main magnet system, attached to one of the first and second part of the motor, constructed and arranged to provide a first magnetic field, substantially perpendicular to the direction of the force; an electrically conductive element attached to the other of the one part of the motor and arranged so as to produce the force by interaction of an electric current carried by the electrically conductive element and the first magnetic field; and a subsidiary magnet system, attached to the one part of the motor and arranged in a Halbach configuration, providing a second magnetic field substantially perpendicular to the first magnetic field.
This arrangement is advantageous since it reduces the size of the back iron required to prevent saturation. The mass of back iron in the actuator will be reduced and will further improve the efficiency of the actuator as the moving mass of the actuator is decreased the force required to produce a given acceleration is decreased. It also increases the k-factor of the actuator (also referred to as the motor constant) which means that the force produced for a given flow of current through the coils is increased. The combination of the effects results in a significant reduction in the current to effect a given acceleration which in turn reduces the amount of heat generated by the coils.
In an embodiment of the present invention, the subsidiary magnet system comprises a first and a second subsidiary magnet and the main magnet system comprises a main magnet, at least a part of which is located between the two subsidiary magnets. The magnets are oriented such that the magnetic polarization of the first subsidiary magnet is substantially anti-parallel to that of the second subsidiary magnet and the magnetic polarization of the main magnet is substantially perpendicular to those of the two subsidiary magnets.
In a yet further embodiment of the present invention, the Lorentz actuator comprises two magnet assemblies. The first magnet assembly comprises a first main magnet system sub-assembly and a first subsidiary magnet system sub-assembly and the second magnet assembly comprises a second main magnet system sub-assembly and a second subsidiary magnet system sub-assembly. At least a part of the electrically conductive element is located between the first and second magnet assemblies.
In a yet further embodiment of the present invention, each magnet assembly comprises first and second main magnets, oriented such that their magnetic polarizations are substantially anti-parallel to each other. The magnet assemblies further comprise first, second and third subsidiary magnets arranged such that at least a portion of the first main magnet is located between the first and second subsidiary magnets and at least a portion of the second main magnet is located between the second and third subsidiary magnets. The electrically conductive element comprises a first part, located between the first main magnet of the first magnet assembly and the first main magnet of the second assembly, and a second part, located between the second main magnet of the first magnet assembly and the second main magnet of the second magnet assembly. The electrically conductive element is arranged such that, when it conducts electric current, the direction of the electric current in the first part is substantially anti-parallel to the direction of the electric current in the second part.
The invention also relates to a motor for use in a lithographic projection apparatus comprising a long stoke motor for producing a force between a first part of the motor and a machine frame of the lithographic projection apparatus over a long range of movement in at least one direction and a Lorentz actuator for producing a force between the first part and a second part of the motor over a short range of movement in at least one direction, wherein the Lorentz actuator includes a main magnet system, attached to one of the first and second part of the motor, constructed and arranged to provide a first magnetic field, substantially perpendicular to the direction of the force; and an electrically conductive element attached to the other of the one part of the motor and arranged so as to produce the force by interaction of an electric current carried by the electrically conductive element and the first magnetic field; and a subsidiary magnet system, attached to the one part of the motor and arranged in a Halbach configuration, constructed and arranged to provide a second magnetic field substantially perpendicular to the first magnetic field.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.